Recently, highs-speed partial plating has been widely applied in silver plating of electronic components and materials such as lead frames. However, direct plating of electronic components and materials usually results in poor adhesion to the substrates. Therefore, conventional high-speed silver plating is carried out after treating these substrates by copper strike plating. One characteristic feature of the plating solution using this process is a very high silver concentration. When a substrate material made of less-noble metal than silver, such as copper or copper alloy, is immersed in the plating solution of high silver concentration, a large amount of silver is deposited by a displacement reaction merely by the immersion of the substrate. The deposited silver layer by displacement on the substrate usually shows extremely poor adhesion, and the poor adhesion could not be improved by successive electroplating. The poor adhesion causes subsequently formed electroplates of silver to scale off or blister or tarnish upon heating, and results in detecting plating. Moreover, in partial plating, even the portions that need not be plated become plated, consuming the expensive silver wastefully. A further disadvantage is the contamination of the plating bath with ions of copper or other less-noble metals that have dissolved out of the substrate by the displacement reaction with silver.
To prevent such harmful deposits of silver by displacement, the inventors of the present invention have proposed a process of pretreatment and a pretreating solution for substrates to be plated with a solution containing thiocarboxylic acid or salts thereof (Japanese Laid Open Patent Application No. 190589/1985), nitrogen containing heterocyclic compounds such as 2,2'-dipyridyl (Japanese Laid Open Patent Application No. 190591/1985) and cyclic compounds whose ring includes a thioureylene radical such as 2-thiobarbituric acid (Japanese patent Publication No. 32318/1989) and other ingredients.
The pretreatment process using the above mentioned solutions requires one additional step by dividing the high-speed silver plating process into pretreating and plating processes.
However, the process provides the following advantages in comparison to the one step process, which adds an agent to the silver plating solution for the prevention of silver deposition by the displacement.
(1) Metal dissolution into silver plating solution by immersing the metallic substrate, which occurs in a very short period of time before the expression of the effects of the agent for preventing the silver deposition by displacement, can be prevented.
(2) Mixing the agent for preventing the silver deposition by displacement into the silver plating solution can be prevented.
(3) Therefore, the mixing of the components in the agent for preventing the silver deposition by displacement into the silver electroplated layer can be prevented.
(4) Harmful action in plating work that may be caused by mixing the agent for preventing the silver deposition by displacement into the silver plating solution can be prevented.
However, it became clear that KCN in the copper strike plating treatment solution is transferred into the pretreating solution and accumulates therein by repeated uptake of a very small amount of KCN from the copper strike plating treatment solution for a long period of time. The preventive effect of the silver deposition by displacement in the pretreating solution declined due to this accumulated KCN. This accumulation of KCN presented a new problem to be solved.
The transferring of KCN cannot be prevented completely, even if the metallic substrate is thoroughly washed with water after copper strike plating treatment.
When the KCN concentration in the pretreating solution becomes more than 25 mg/l, the thin film of the agent on the substrate for preventing the silver deposition by displacement is partially damaged or scaled off by KCN or a combination of the agent with KCN loses the preventive effect against the silver deposition by displacement. The effect can not be recovered even if the agent for preventing the silver deposition by displacement is replenished. Therefore, satisfactory quality silver plating cannot be obtained by high-speed silver plating due to the deposition of silver by displacement in the damaged portions.
Therefore, operation of plating has to be stopped occasionally to replace the pretreating solution or the high-speed silver plating solution is under serious condition.
Generally, the conventional copper strike plating solution approximately composed of CuCN: 90 g/l, KCN: 145 g/l and KOH: 90 g/l. The approximate composition of the normal high-speed silver plating solution is KAg(CN).sub.2 : 130 g/l, K.sub.2 HPO.sub.4 : 100 g/l and pH range of 8 to 9. Both compositions are alkaline. Therefore, pretreating solution between the two solutions is generally used under alkaline conditions containing 0.1 to 20 g/l of KOH or NaOH, or adding a pH buffer such as K.sub.2 HPO.sub.4.
The object of the present invention is to provide a pretreating solution for silver plating, which does not reduce the effect for preventing the silver deposition by displacement, even if operation of plating is repeated for a long period of time.
In other words, the purpose of the invention is to provide the pretreating solution for silver plating, which has a long effective life for preventing the silver deposition without replenishment.
One further purpose of the present invention is to provide a pretreating solution for silver plating, which enhances adhesion of the silver plating layer on metallic substrate surfaces.
Another purpose of the invention is to provide a pretreating solution for silver plating, which can reduce the loss of silver in silver plating solution.
Yet another purpose of the present invention is to provide a silver plating treatment process using such a pretreating solution for silver plating.